public abstract class AbstractAggregatingMessageGroupProcessor
              implements MessageGroupProcessor {

    protected Map<String, Object> aggregateHeaders(MessageGroup group) {
        // default implementation exists
    }

    protected abstract Object aggregatePayloads(MessageGroup group, Map<String, Object> defaultHeaders);

}

public AbstractCorrelatingMessageHandler(MessageGroupProcessor processor, MessageGroupStore store,
        CorrelationStrategy correlationStrategy, ReleaseStrategy releaseStrategy) {
    ...
    this.correlationStrategy = correlationStrategy == null ?
        new HeaderAttributeCorrelationStrategy(IntegrationMessageHeaderAccessor.CORRELATION_ID) : correlationStrategy;
    this.releaseStrategy = releaseStrategy == null ? new SimpleSequenceSizeReleaseStrategy() : releaseStrategy;
    ...
}

public interface ReleaseStrategy {

  boolean canRelease(MessageGroup group);

}

public class MyReleaseStrategy {

    @ReleaseStrategy
    public boolean canMessagesBeReleased(List<Message<?>>) {...}
}

public class MyReleaseStrategy {

    @ReleaseStrategy
    public boolean canMessagesBeReleased(List<String>) {...}
}

<int:aggregator input-channel="aggregate"
    output-channel="out" message-store="store" release-strategy="releaser" />

<bean id="store" class="org.springframework.integration.store.SimpleMessageStore">
    <property name="messageGroupFactory">
        <bean class="org.springframework.integration.store.SimpleMessageGroupFactory">
            <constructor-arg value="BLOCKING_QUEUE"/>
        </bean>
    </property>
</bean>

<bean id="releaser" class="SimpleSequenceSizeReleaseStrategy" />

public interface CorrelationStrategy {

  Object getCorrelationKey(Message<?> message);

}

...->aggregator1-> ... ->aggregator2-> ...

Found one Java-level deadlock:
=============================
"t2":
  waiting for ownable synchronizer 0x000000076c1cbfa0, (a java.util.concurrent.locks.ReentrantLock$NonfairSync),
  which is held by "t1"
"t1":
  waiting for ownable synchronizer 0x000000076c1ccc00, (a java.util.concurrent.locks.ReentrantLock$NonfairSync),
  which is held by "t2"

<channel id="inputChannel"/>

<int:aggregator id="myAggregator"                          (1)
        auto-startup="true"                                (2)
        input-channel="inputChannel"                       (3)
        output-channel="outputChannel"                     (4)
        discard-channel="throwAwayChannel"                 (5)
        message-store="persistentMessageStore"             (6)
        order="1"                                          (7)
        send-partial-result-on-expiry="false"              (8)
        send-timeout="1000"                                (9)

        correlation-strategy="correlationStrategyBean"     (10)
        correlation-strategy-method="correlate"            (11)
        correlation-strategy-expression="headers['foo']"   (12)

        ref="aggregatorBean"                               (13)
        method="aggregate"                                 (14)

        release-strategy="releaseStrategyBean"             (15)
        release-strategy-method="release"                  (16)
        release-strategy-expression="size() == 5"          (17)

        expire-groups-upon-completion="false"              (18)
        empty-group-min-timeout="60000"                    (19)

        lock-registry="lockRegistry"                       (20)

        group-timeout="60000"                              (21)
        group-timeout-expression="size() ge 2 ? 100 : -1"  (22)
        expire-groups-upon-timeout="true"                  (23)

        scheduler="taskScheduler" >                        (24)
            <expire-transactional/>                        (25)
            <expire-advice-chain/>                         (26)
</aggregator>

<int:channel id="outputChannel"/>

<int:channel id="throwAwayChannel"/>

<bean id="persistentMessageStore" class="org.springframework.integration.jdbc.store.JdbcMessageStore">
    <constructor-arg ref="dataSource"/>
</bean>

<bean id="aggregatorBean" class="sample.PojoAggregator"/>

<bean id="releaseStrategyBean" class="sample.PojoReleaseStrategy"/>

<bean id="correlationStrategyBean" class="sample.PojoCorrelationStrategy"/>

<aggregator input-channel="input" method="sum" output-channel="output">
    <beans:bean class="org.foo.PojoAggregator"/>
</aggregator>

public class PojoAggregator {

  public Long add(List<Long> results) {
    long total = 0l;
    for (long partialResult: results) {
      total += partialResult;
    }
    return total;
  }
}

public class PojoReleaseStrategy {
...
  public boolean canRelease(List<Long> numbers) {
    int sum = 0;
    for (long number: numbers) {
      sum += number;
    }
    return sum >= maxValue;
  }
}

public class PojoCorrelationStrategy {
...
  public Long groupNumbersByLastDigit(Long number) {
    return number % 10;
  }
}

public class Waiter {
  ...

  @Aggregator  (1)
  public Delivery aggregatingMethod(List<OrderItem> items) {
    ...
  }

  @ReleaseStrategy  (2)
  public boolean releaseChecker(List<Message<?>> messages) {
    ...
  }

  @CorrelationStrategy  (3)
  public String correlateBy(OrderItem item) {
    ...
  }
}

@ServiceActivator(inputChannel = "aggregatorChannel")
@Bean
public MessageHandler aggregator(MessageGroupStore jdbcMessageGroupStore) {
     AggregatingMessageHandler aggregator =
                       new AggregatingMessageHandler(new DefaultAggregatingMessageGroupProcessor(),
                                                 jdbcMessageGroupStore);
     aggregator.setOutputChannel(resultsChannel());
     aggregator.setGroupTimeoutExpression(new ValueExpression<>(500L));
     aggregator.setTaskScheduler(this.taskScheduler);
     return aggregator;
}